Shutdown system for a nuclear steam supply system

ABSTRACT

A nuclear steam supply system having a shutdown system for removing residual decay heat generated by a nuclear fuel core. The steam supply system may utilize gravity-driven primary coolant circulation through hydraulically interconnected reactor and steam generating vessels forming the steam supply system. The shutdown system may comprise primary and secondary coolant systems. The primary coolant cooling system may include a jet pump comprising an injection nozzle disposed inside the steam generating vessel. A portion of the circulating primary coolant is extracted, pressurized and returned to the steam generating vessel to induce coolant circulation under reactor shutdown conditions. The extracted primary coolant may further be cooled before return to the steam generating vessel in some operating modes. The secondary coolant cooling system includes a pumped and cooled flow circuit operating to circulate and cool the secondary coolant, which in turn extracts heat from and cools the primary coolant.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 14/433,394 filed Apr. 3, 2015, which is a continuation-in-partof U.S. patent application Ser. No. 14/620,390 filed Feb. 12, 2015,which is a 371 National Stage entry of PCT International PatentApplication No. PCT/US2013/063405 filed Oct. 4, 2013, which claims thebenefit of U.S. Provisional Patent Application Ser. No. 61/709,436 filedOct. 4, 2012, and is a continuation-in-part of PCT International PatentApplication No. PCT/US2013/054961 filed Aug. 14, 2013, which claimspriority to U.S. Provisional Patent Application Ser. No. 61/683,021,filed Aug. 14, 2012; the entireties of which are all incorporated hereinby reference.

FIELD OF THE INVENTION

The present invention relates to nuclear steam supply systems, and moreparticularly to a shutdown system for a nuclear steam supply systemusable for cooling primary and secondary coolant.

BACKGROUND OF THE INVENTION

For starting up a nuclear steam supply system in a typical pressurizedwater reactor (PWR), it is necessary to heat the reactor coolant waterto an operating temperature, which is known in the art as the no-loadoperating temperature of the reactor coolant water. Furthermore, inconventional nuclear steam supply systems it is necessary to ensure fullflow through the coolant loop and the core. This is necessary to ensurethat a completely turbulated flow across the fuel core exists as thecontrol rods are being withdrawn in order to avoid localized heating andboiling, and to ensure that the reactivity of water is in the optimalrange during start-up and during normal operation.

In the present state of the art, the desired start-up condition isachieved by the use of the reactor coolant pump whose primary functionis to circulate coolant through the reactor core during normal operatingconditions. In normal operation, the substantial frictional heatproduced by the reactor coolant pumps is removed by external coolingequipment (heat exchangers) to maintain safe operating temperature.However, during start-up external cooling is disabled so that thefrictional heat can be directly transferred to the reactor coolantwater, enabling it to reach no-load operating temperature. As thereactor coolant water is being heated, the pressure in the reactorcoolant loop is increased using a bank of internal heaters whichevaporates some reactor coolant water and increases the pressure in thereactor coolant system by maintaining a two-phase equilibrium.

The above process for heating the reactor water inventory duringstart-up is not available in a passively safe nuclear steam supplysystem. This is because such a passively safe nuclear steam supplysystem does not include or require any pumps, and thus the use of thefrictional heat is unavailable for heating the reactor water inventory.Thus, a need exists for a start-up system for heating the reactor waterinventory in a passively safe nuclear steam supply system.

According to another aspect of PWRs, it is desirable to provide ashutdown system for a nuclear steam supply system to cool primary andsecondary coolant in order to bring the reactor from a hot full powerstate to a cold and shutdown state in a safe and controlled manner whichprotects the reactor and steam supply system from potential damageassociated thermal and/or pressure transients.

SUMMARY OF THE INVENTION

The present disclosure provides an improved shutdown system for anuclear steam supply system. The shutdown system may include a primarycoolant cooling system and a secondary coolant cooling systems. Bothcooling systems may be operated in tandem and cooperation to cool theprimary coolant, which in turn removes and rejects residual decay heatproduced by the nuclear fuel core during reactor shutdown conditions. Inone embodiment, as further described herein, the primary and secondarycoolant cooling systems may be operated in sequential stages or phasesto gradually and safely bring the reactor from a hot full power state toa cold and shutdown state.

In one embodiment, a nuclear steam supply system with shutdown coolingsystem includes: a reactor vessel having an internal cavity; a reactorcore comprising nuclear fuel disposed within the internal cavity andoperable to heat a primary coolant; a steam generating vessel fluidlycoupled to the reactor vessel; a riser pipe positioned within the steamgenerating vessel and fluidly coupled to the reactor vessel; a primarycoolant loop formed within the reactor vessel and the steam generatingvessel, the primary coolant loop being configured for circulatingprimary coolant through the reactor vessel and steam generating vessel;and a primary coolant cooling system. The primary coolant systemincludes: an intake conduit having an inlet fluidly coupled to theprimary coolant loop; a pump fluidly coupled to the intake conduit, thepump configured and operable to extract and pressurize primary coolantfrom the primary coolant loop and discharge the pressurized primarycoolant through an injection conduit; a Venturi injection nozzle havingan inlet fluidly coupled to the injection conduit and positioned withinthe riser pipe to inject pressurized primary coolant into the riser pipefrom the pump; and a heat exchanger configured and operable to cool theextracted primary coolant.

In another embodiment, a nuclear steam supply system with shutdowncooling system includes: a reactor vessel having an internal cavity; areactor core comprising nuclear fuel disposed within the internal cavityand operable to heat a primary coolant; a steam generating vesselfluidly coupled to the reactor vessel and containing a secondary coolantfor producing steam to operate a steam turbine, the steam generatingvessel including a superheater section and a steam generator section; ariser pipe positioned inside the steam generating vessel and fluidlycoupled to the reactor vessel; a primary coolant flow loop formed withinthe reactor vessel and the steam generating vessel, the primary coolantflow loop being configured and operable for circulating primary coolantthrough the reactor vessel and steam generating vessel; a primarycoolant cooling system; and a secondary coolant cooling system. Theprimary coolant cooling system includes: a first pump having an inletfluidly coupled to the primary coolant flow loop, the first pumpconfigured and operable to extract and pressurize a portion of theprimary coolant from the primary coolant loop; a Venturi injectionnozzle having an inlet fluidly coupled to a discharge of the first pumpand positioned inside the riser pipe in the steam generating vessel, theinjection nozzle receiving and injecting the pressurized portion of theprimary coolant into the riser pipe from the pump; and a first heatexchanger configured and operable to cool the extracted primary coolantprior to injecting the pressurized portion of the primary coolant. Thesecondary coolant cooling system includes: a steam bypass condenserhaving an inlet fluidly coupled to the superheater section of the steamgenerator vessel for receiving and cooling secondary coolant in a steamphase; a second heat exchanger having an inlet fluidly coupled to thesteam generator section of the steam generating vessel for receiving andcooling secondary coolant in a liquid phase; and a second pump having aninlet fluidly coupled to the steam bypass condenser and the second heatexchanger, the second pump configured and operable to pressurize andcirculate secondary coolant through the steam generator in a secondarycoolant flow loop. The secondary coolant cooling system is configured tocool secondary coolant in either the steam or liquid phase.

In another embodiment, a nuclear steam supply system with shutdowncooling system includes: a reactor vessel having an internal cavity; avertically elongated reactor core comprising nuclear fuel disposedwithin the internal cavity and operable to heat a primary coolant; avertically elongated steam generating vessel fluidly coupled to thereactor vessel and containing a secondary coolant for producing steam tooperate a steam turbine, the steam generating vessel including asuperheater section and a steam generator section; a verticallyelongated riser pipe positioned inside the steam generating vessel andfluidly coupled to the reactor vessel; a primary coolant flow loopformed within the reactor vessel and the steam generating vessel, theprimary coolant flow loop being configured and operable for circulatingprimary coolant through the reactor vessel and steam generating vessel;a secondary coolant flow loop formed outside of the reactor vessel andsteam generating vessel, the secondary coolant flow loop beingconfigured and operable for circulating secondary coolant through thesteam generating vessel; and a Venturi jet pump disposed inside theriser pipe of the steam generating vessel, the jet pump including aninjection nozzle fluidly coupled to the primary coolant flow loop. Thejet pump receives and injects a portion of the primary coolant into theriser pipe which draws and mixes primary coolant from the reactor vesselwith the injected portion of the primary coolant in the riser pump tocirculate primary coolant through the primary coolant flow loop.

A method for removing residual decay heat from a nuclear reactor fuelcore under shutdown conditions is provided. The method includes:providing a steam generating vessel hydraulically coupled to a reactorvessel housing a nuclear fuel core; circulating a primary coolantthrough a primary coolant flow loop formed inside and between the steamgenerating vessel and reactor vessel; extracting a portion of theprimary coolant from the primary coolant flow loop; pressurizing theextracted portion of the primary coolant; injecting the extractedportion of the primary coolant into a riser pipe disposed inside thesteam generating vessel through a Venturi injection nozzle; and drawingprimary coolant from the reactor vessel into the riser pipe using theinjection nozzle. In one embodiment, the method further includes coolingthe extracted portion of the primary coolant prior to injecting theextracted portion of the primary coolant into the riser pipe. In oneembodiment, the cooling step is performed using a first tubular heatexchanger. The tubular heat exchanger may be a dual purpose heatexchanger configured for either cooling the primary coolant during steamsupply system shutdown or heating the primary coolant during steamsupply system startup.

The present invention further provides an improved nuclear steam supplysystem and start-up sub-system therefor that overcomes the deficienciesof the foregoing existing arrangements. The present invention alsoprovides an improved method of heating a primary coolant in a nuclearsteam supply system to a no load operating temperature.

In one aspect, the invention can be a nuclear steam supply systemcomprising: a reactor vessel having an internal cavity, a reactor corecomprising nuclear fuel disposed within the internal cavity; a steamgenerating vessel fluidly coupled to the reactor vessel; a riser pipepositioned within the steam generating vessel and fluidly coupled to thereactor vessel; a primary coolant at least partially filling a primarycoolant loop formed within the reactor vessel and the steam generatingvessel; and a start-up sub-system comprising: an intake conduit havingan inlet located in the primary coolant loop; a pump fluidly coupled tothe intake conduit for pumping a portion of the primary coolant from theprimary coolant loop through the intake conduit and into an injectionconduit; at least one heating element for heating the portion of theprimary coolant to form a heated portion of the primary coolant; and aninjection nozzle fluidly coupled to the injection conduit and positionedwithin the riser pipe for injecting the heated portion of the primarycoolant into the riser pipe.

In another aspect, the invention can be a nuclear steam supply systemcomprising: a reactor vessel having an internal cavity, a reactor corecomprising nuclear fuel disposed within the internal cavity; a steamgenerating vessel fluidly coupled to the reactor vessel; a primarycoolant loop formed within the reactor vessel and the steam generatingvessel, a primary coolant in the primary coolant loop; and a start-upsub-system fluidly coupled to the primary coolant loop, the start-upsub-system configured to: (1) receive a portion of the primary coolantfrom the primary coolant loop; (2) heat the portion of the primarycoolant to form a heated portion of the primary coolant; and (3) injectthe heated portion of the primary coolant into the primary coolant loop.

In yet another aspect, the invention can be a method of heating aprimary coolant to a no-load operating temperature in a nuclear steamsupply system, the method comprising: a) filling a primary coolant loopwithin a reactor vessel and a steam generating vessel that are fluidlycoupled together with a primary coolant; b) drawing a portion of theprimary coolant from the primary coolant loop and into a start-upsub-system; c) heating the portion of the primary coolant within thestart-up sub-system to form a heated portion of the primary coolant; andd) injecting the heated portion of the primary coolant into the primarycoolant loop.

In a further aspect, the invention can be a method of starting up anuclear steam supply system, the method comprising: a) at leastpartially filling a primary coolant loop within a reactor vessel and asteam generating vessel that are fluidly coupled together with a primarycoolant, wherein the primary coolant loop comprises a riser pipe in thesteam generating vessel; b) drawing a portion of the primary coolantfrom the primary coolant loop and into a start-up sub-system; c) heatingthe portion of the primary coolant within the start-up sub-system toform a heated portion of the primary coolant; and d) introducing theheated portion of the primary coolant into the riser pipe of the steamgenerating vessel.

Further areas of applicability of the present invention will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples, whileindicating the preferred embodiment of the invention, are intended forpurposes of illustration only and are not intended to limit the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the exemplary embodiments of the present invention willbe described with reference to the following drawings, where likeelements are labeled similarly, and in which:

FIG. 1 is front view of a nuclear steam supply system including areactor vessel, a steam generating vessel and a start-up sub-system inaccordance with an embodiment of the present invention;

FIG. 2 is an elevation cross-sectional view of the reactor vessel ofFIG. 1;

FIG. 3 is an elevation cross-sectional view of the bottom portion of thesteam generating vessel of FIG. 1;

FIG. 4 is an elevation cross-sectional view of the top portion of thesteam generating vessel of FIG. 1;

FIG. 5A is a close-up view of the reactor vessel and a portion of thesteam generating vessel of FIG. 1 illustrating the location of an intakeconduit of the start-up sub-system in accordance with a first embodimentof the present invention;

FIG. 5B is the close-up view of FIG. 5A illustrating the location of theintake conduit of the start-up sub-system in accordance with a secondembodiment of the present invention;

FIG. 5C is the close-up view of FIG. 5A illustrating the location of theintake conduit of the start-up sub-system in accordance with a thirdembodiment of the present invention;

FIG. 6 is a close-up view of area VI of FIG. 1;

FIG. 7 is a schematic flow diagram illustrating the connection betweenthe start-up sub-system and the reactor vessel;

FIG. 8 is a graph illustrating the primary coolant pressure vs. theprimary coolant temperature;

FIG. 9 is a schematic flow diagram illustrating a nuclear steam supplyshutdown system in a first phase or stage of operation;

FIG. 10 is a schematic flow diagram illustrating a nuclear steam supplyshutdown system in a subsequent second phase or stage of operation; and

FIG. 11 is a graph illustrating the decay heat load versus time of ashutdown reactor core.

All drawings are schematic and not necessarily to scale.

DETAILED DESCRIPTION OF THE INVENTION

The features and benefits of the invention are illustrated and describedherein by reference to exemplary embodiments. This description ofexemplary embodiments is intended to be read in connection with theaccompanying drawings, which are to be considered part of the entirewritten description. In the description of embodiments disclosed herein,any reference to direction or orientation is merely intended forconvenience of description and is not intended in any way to limit thescope of the present invention. Relative terms such as “lower,” “upper,”“horizontal,” “vertical,”, “above,” “below,” “up,” “down,” “top” and“bottom” as well as derivative thereof (e.g., “horizontally,”“downwardly,” “upwardly,” etc.) should be construed to refer to theorientation as then described or as shown in the drawing underdiscussion. These relative terms are for convenience of description onlyand do not require that the apparatus be constructed or operated in aparticular orientation. Terms such as “attached,” “affixed,”“connected,” “coupled,” “interconnected,” and similar refer to arelationship wherein structures are secured or attached to one anothereither directly or indirectly through intervening structures, as well asboth movable or rigid attachments or relationships, unless expresslydescribed otherwise. Accordingly, the disclosure expressly should not belimited to such exemplary embodiments illustrating some possiblenon-limiting combination of features that may exist alone or in othercombinations of features.

Referring first to FIG. 1, a nuclear steam supply system 100 isillustrated in accordance with an embodiment of the present invention.Although described herein as being a nuclear steam supply system, incertain embodiments the system may be generally referred to herein as asteam supply system. The inventive nuclear steam supply system 100 istypically used in a nuclear pressurized water reactor and generallycomprises a reactor vessel 200, a steam generating vessel 300 and astart-up sub-system 500. Of course, the nuclear steam supply system 100can have uses other than for nuclear pressurized water reactors as canbe appreciated.

During normal operation of the nuclear steam supply system 100, aprimary coolant flows through a primary coolant loop 190 within thereactor vessel 200 and the steam generating vessel 300. This primarycoolant loop 190 is depicted with arrows in FIG. 1. Specifically, theprimary coolant flows upwardly through a riser column 224 in the reactorvessel 200, from the reactor vessel 200 to the steam generating vessel300 through a fluid coupling 270, upwardly through a riser pipe 337 inthe steam generating vessel 300 to a top of the steam generating vessel300 (i.e., to a pressurizer 380), and then downwardly through tubes 332(see FIGS. 3 and 4) in a tube side 304 of the steam generating vessel300, from the steam generating vessel 300 to the reactor vessel 200through the fluid coupling 270, downwardly through a downcomer 222 ofthe reactor vessel 200, and then back from the downcomer 222 of thereactor vessel 200 to the riser column 224 of the reactor vessel 200.The primary coolant continues to flow along this primary coolant loop190 as desired without the use of any pumps during normal operation ofthe nuclear steam supply system 100.

It should be appreciated that in certain embodiments the primary coolantloop 190 is filled or partially filled with the primary coolant when thenuclear steam supply system 100 is shut down and not operating. Byfilled it may mean that the entire primary coolant loop 190 iscompletely filled with the primary coolant, or that the primary coolantloop 190 is almost entirely filled with the primary coolant with someroom for air which leaves space for the addition of more primary coolantif desired or the expansion of the primary coolant as it heats up duringthe start-up procedures discussed below. In certain embodiments, beforestart-up the primary coolant is static in the primary coolant loop 190in that there is no flow of the primary coolant along the primarycoolant loop. However, during a start-up procedure utilizing thestart-up sub-system 500 discussed in detail below, the primary coolantis heated and caused to flow through the primary coolant loop 190 andeventually is able to flow through the primary coolant loop 190passively and unaided by any pumps due to the physics concept ofthermosiphon flow.

Before nuclear fuel within the reactor core engages in a fission chainreaction to produce heat, a start-up process using the start-upsub-system 500 takes place to heat the primary coolant to a no-loadoperating temperature, as discussed in more detail below. During normaloperation of the nuclear steam supply system 100, the primary coolanthas an extremely high temperature due to its flowing through the reactorcore. Specifically, nuclear fuel in the reactor vessel 200 engages inthe fission chain reaction, which produces heat and heats the primarycoolant as the primary coolant flows through the reactor core of thereactor vessel 200. This heated primary coolant is used to phase-changea secondary coolant from a liquid into steam as discussed below.

While the primary coolant is flowing through the primary coolant loop190 during normal operation, the secondary coolant is flowing through asecond coolant loop. Specifically, the secondary coolant is introducedinto the shell side 305 (FIGS. 3 and 4) of the steam generating vessel300 at the secondary coolant in location indicated in FIG. 1. Thesecondary coolant then flows through the shell side 305 (FIGS. 3 and 4)of the steam generating vessel 300 where it is heated by heat transferfrom the primary coolant. The secondary coolant is converted into steamdue to the heat transfer, and the steam flows from the steam generatingvessel 300 to a turbine 900 as indicated in FIG. 1. The turbine 900drives an electric generator 910 which is connected to the electricalgrid for power distribution. The steam then travels from the turbine 900to a condenser (not illustrated) whereby the steam is cooled down andcondensed to form condensate. Thus, the condenser converts the steamback to a liquid condensate (i.e., the secondary coolant) so that it canbe pumped back into the steam generator 300 at the secondary coolantinlet location and repeat its flow through the flow path discussed aboveand be converted back to steam.

In certain embodiments both the primary coolant and the secondarycoolant may be water, such as demineralized water. However, theinvention is not to be so limited and other liquids or fluids can beused in certain other embodiments, the invention not being limited tothe material of the primary and secondary coolants unless so claimed.

The primary coolant continues to flow through the primary coolant loopand the secondary coolant continues to flow in the second coolant loopduring normal operation of the nuclear steam supply system 100. Thegeneral provision and operation of the gravity-driven nuclear steamsupply system 100 and details of the associated components is describedin detail in International Application No. PCT/US13/38289, filed on Apr.25, 2013, the entirety of which is incorporated herein by reference.

Referring to FIGS. 1-4, the general details of the components and theoperation of the nuclear steam supply system 100, and specifically ofthe reactor vessel 200 and the steam generating vessel 300, will bedescribed. In the exemplified embodiment, the reactor vessel 200 and thesteam generating vessel 300 are vertically elongated and separatecomponents which hydraulically are closely coupled, but are discretevessels in themselves that are thermally isolated except for theexchange of primary coolant (i.e. reactor coolant) flowing between thevessels in the fluid coupling 270 of the primary coolant loop 190 asdiscussed above. In one non-limiting embodiment, each of the reactorvessel 200 and the steam generating vessel 300 may be made of acorrosion resistant metal such as stainless steel, although othermaterials of construction are possible.

Referring to FIGS. 1 and 2 concurrently, the reactor vessel 200 will befurther described. The reactor vessel 200 in one non-limiting embodimentis an ASME code Section III, Class 1 thick-walled cylindrical pressurevessel comprised of a cylindrical sidewall shell 201 with an integrallywelded hemispherical bottom head 203 and a removable hemispherical tophead 202. The shell 201 defines an internal cavity 208 configured forholding the reactor core which comprises the nuclear fuel. Specifically,the reactor vessel 200 includes a cylindrical reactor shroud 220 whichcontains the reactor core defined by a fuel cartridge 230 (i.e., nuclearfuel). The reactor shroud 220 transversely divides the shell portion ofthe reactor vessel into two concentrically arranged spaces: (1) an outerannulus 221 defining the annular downcomer 222 for primary coolantentering the reactor vessel which is formed between the outer surface ofthe reactor shroud 220 and an inner surface 206 of the shell 201; and(2) a passageway 223 defining the riser column 224 for the primarycoolant leaving the reactor vessel 200 heated by fission in the reactorcore.

The reactor shroud 220 is elongated and extends in an axial directionalong a vertical axis A-A of the reactor vessel 200. The reactor shroud220 includes an open bottom end 225 and a closed top end 226. In oneembodiment, the open bottom end 225 of the reactor shroud 220 isvertically spaced apart by a distance from the bottom head 203 of thereactor vessel 200 thereby forming a bottom flow plenum 228 between thebottom end 225 of the reactor shroud 220 and the bottom head 203 of thereactor vessel 200. As will be discussed in more detail below, duringflow of the primary coolant through the primary coolant loop 190, thebottom flow plenum 228 collects the primary coolant from the annulardowncomer 222 and directs the primary coolant flow into the inlet of theriser column 224 formed by the open bottom end 225 of the reactor shroud220.

In certain embodiments, the reactor shroud 220 is a double-walledcylinder which may be made of a corrosion resistant material, such aswithout limitation stainless steel. This double-wall construction of thereactor shroud 220 forms an insulated structure designed to retard theflow of heat across it and forms a smooth vertical riser column 224 forupward flow of the primary coolant heated by the fission in the fuelcartridge 230 (“core”), which is preferably located at the bottomextremity of the shroud 220 in one embodiment as shown in FIG. 2. Thevertical space above the fuel cartridge 230 in the reactor shroud 220may also contain interconnected control rod segments along with a set of“non-segmental baffles” that serve to protect them from flow inducedvibration during reactor operations. The reactor shroud 220 is laterallysupported by the reactor vessel by support members 250 to prevent damagefrom mechanical vibrations that may induce failure from metal fatigue.

In certain embodiments, the fuel cartridge 230 is a unitary autonomousstructure containing upright fuel assemblies, and is situated in aregion of the reactor vessel 200 that is spaced above the bottom head203 so that a relatively deep plenum of water lies underneath the fuelcartridge 230. The fuel cartridge 230 is insulated by the reactor shroud220 so that a majority of the heat generated by the fission reaction inthe nuclear fuel core is used in heating the primary coolant flowingthrough the fuel cartridge 230 and adjoining upper portions of the risercolumn 224. In certain embodiments, the fuel cartridge 230 is an opencylindrical structure including cylindrically shaped sidewalls, an opentop, and an open bottom to allow the primary coolant to flow upwardcompletely through the cartridge (see directional flow arrows, describedin detail above with specific reference to FIG. 1). In one embodiment,the sidewalls of the fuel cartridge 230 may be formed by multiplearcuate segments of reflectors which are joined together by suitablemeans. The open interior of the fuel cartridge 230 may be filled with asupport grid for holding the nuclear fuel rods and for insertion ofcontrol rods into the core to control the fission reaction as needed.

In the interconnecting space between the reactor vessel 200 and thesteam generating vessel 300 there is a fluid coupling 270 that comprisesan inner flow path 271 and an outer flow path 272 that concentricallysurrounds the inner flow path 271. As will be discussed in more detailbelow, during flow of the primary coolant the primary coolant flowsupwardly within the riser column 224 and through the inner flow path 271of the fluid coupling 270 to flow from the reactor vessel 200 to thesteam generating vessel 300. After the primary coolant gets to the topof the steam generating vessel 300, the primary coolant begins adownward flow through the steam generating vessel 300 and then flowsthrough the outer flow path 272 from the steam generating vessel 300 andinto the downcomer 222 of the reactor vessel 200. Again, this flow pathwill be described in more detail below.

Turning now to FIGS. 1, 3 and 4 concurrently, the details of the steamgenerating vessel 300 will be described in more detail. In certainembodiments, the steam generating vessel 300 includes a preheatersection 320, a steam generator section 330, a superheater section 340and a pressurizer 380. However, the invention is not to be so limitedand one or more of the sections of the steam generating vessel 300 maybe omitted in certain other embodiments. Specifically, in certainembodiments the preheater section 320 may be omitted, or may itself beconsidered a part of the steam generator section 330. A steam bypassloop 303 may be provided (see, e.g. FIG. 9) to route saturated steamfrom the steam generator section 330 to the superheater section 340around the intermediate tubesheet structure as shown. As discussedabove, it is within the steam generator vessel 300 that the secondarycoolant that is flowing through the shell side 305 of the steamgenerator vessel 300 is converted from a liquid (i.e., secondary coolantinlet illustrated in FIG. 1) to a superheated steam that is sent to theturbine 900 (FIG. 1) for electricity generation via generator 910. Thesecondary coolant flows in the second coolant loop through the shellside of the steam generating vessel 300, out to the turbine 900, fromthe turbine 900 to a condenser, and then back into the shell side of thesteam generating vessel 300.

In the exemplified embodiment, each of the preheater 320, the steamgenerator 330, and the superheater 350 are tubular heat exchangershaving a tube side 304 and a shell side 305. The tube side 304 of thetubular heat exchangers include a tube bundle comprising a plurality ofparallel straight tubes 332 and tubesheets 333 disposed at theextremities or ends of each tube bundle that support the tubes. In theexemplified embodiment, only two tubes 332 are illustrated to avoidclutter. However, in actual use tens, hundreds or thousands of tubes 332can be positioned within each of the sections of the steam generatingvessel 300. In certain embodiments, a bottom-most one of the tubesheets333A is located in the preheater section 320 or in the steam generatorsection 330. This bottom-most tubesheet 333A will be discussed in moredetail below with regard to a location of injection from the start-upsub-system 500 in one exemplified embodiment.

As noted above, in one embodiment the preheater section 320 can beconsidered as a part of the steam generator section 330. In suchembodiments the steam generator section 330 and the superheater section350 can be considered as stacked heat exchangers such that thesuperheater section 350 is disposed above the steam generator section330. In certain embodiments, the preheater section 320, steam generatorsection 330, and superheater section 350 are positioned to form aparallel counter-flow type heat exchanger arrangement in which thesecondary coolant (Rankine cycle) flows in an opposite, but paralleldirection to the primary coolant (see FIGS. 3 and 4). Specifically, thearrows labeled A indicate the flow direction of the primary coolantthrough the riser pipe 337 that is positioned within the steamgenerating vessel 300, the arrows labeled B indicate the flow directionof the primary coolant through the tubes 332 of the steam generatingvessel 300, and the arrows labeled C indicate the flow direction of thesecondary coolant through the shell side 305 of the steam generatingvessel 300. The trio of the foregoing tubular heat exchangers (i.e.preheater, steam generator, and superheater) are hydraulically connectedin series on both the tube side 304 (primary coolant) and the shell side305 (the secondary coolant forming the working fluid of the RankineCycle which changes phase from liquid to superheated gas).

In the exemplified embodiment, the steam generating vessel 300 includesa top 310, a bottom 311, an axially extending cylindrical shell 312, andthe internal riser pipe 337 which is concentrically aligned with theshell 312 and in the exemplified embodiment lies on a centerline C-C ofthe steam generating vessel 300. The tubes 332 are circumferentiallyarranged around the outside of the riser pipe 337 between the riser pipe337 and the shell 312 in sections of the steam generating vessel 300which include the preheater 320, the steam generator 330, and thesuperheater 350. In one embodiment, the riser pipe 337 extendscompletely through all of the tubesheets 333 associated with thepreheater 320, the steam generator 330, and the superheater 350 from thetop of the steam generating vessel 300 to the bottom to form a part ofthe continuous primary coolant loop 190 between the reactor vessel 200and the steam generating vessel 300 all the way to the pressurizer 380.

The fluid coupling 270 includes an inner flowpath 371 and an outerflowpath 372 on the steam generating vessel 300 side of the fluidcoupling 270. The inner flowpath 371 is fluidly coupled to the innerflow path 271 and the outer flowpath 372 is fluidly coupled to the outerflowpath 272. Thus, via these operable couplings the steam generatingvessel 300 is fluidly coupled to the reactor vessel 200 to complete theprimary coolant loop 190 for flow of the primary coolant through boththe reactor vessel 200 and the steam generating vessel 300. An annularspace is formed between the riser pipe 337 and the shell 312, whichforms a bottom plenum 338. The bottom plenum 338 collects and channelsthe primary coolant from the steam generating vessel 300 back to thereactor vessel 200 via the outer flow paths 272, 372. Thus, in theexemplified embodiment the primary coolant flows from the reactor vessel200 to the steam generating vessel 300 through the inner flow paths 271,371 and the primary coolant flows from the steam generating vessel 300to the reactor vessel 200 through the outer flow paths 272, 372.However, the invention is not to be so limited and in other embodimentsthe use of the flow paths 271, 272, 371, 372 can be reversed

The superheater 350 is topped by a pressurizer 380 as shown in FIGS. 1and 4, which is in fluid communication with both the top or outlet ofthe riser pipe 337 and the inlet to the tubes 332 of the superheater350. In one embodiment, the pressurizer 380 is mounted directly to theshell 312 of the steam generating vessel 300 and forms a top head 336aon the shell. In one embodiment, the pressurizer has a domed orhemispherical head and may be welded to the shell 312, or alternativelybolted in other possible embodiments. The pressurizer 380 forms an upperplenum which collects reactor primary coolant rising through riser pipe337 and distributes the primary coolant from the riser pipe 337 to thesuperheater tubes 332. In certain embodiments, the pressurizer 380includes a heating/quenching element 38. (i.e. water/steam) for pressurecontrol of the reactor primary coolant.

Shown schematically in FIG. 4, the heating/quenching element 383 iscomprised of a bank of electric heaters which are installed in thepressurizer section that serve to increase the pressure by boiling someof the primary coolant and creating a steam bubble that resides at thetop of the pressurizer near the head (above the liquid/gas interface 340represented by the dashed line). A water spray column 384 is locatednear the top head 336a of the pressurizer 380 which sprays water intothe steam bubble thereby condensing the steam and reducing the size ofthe steam bubble. The increase/decrease in size of the steam bubbleserves to increase/decrease the pressure of the primary coolant insidethe reactor coolant system. In one exemplary embodiment, arepresentative primary coolant pressure maintained by the pressurizer380 and the heating/quenching element 383 may be without limitationabout 2,250 psi. In alternative embodiments, as noted above, theliquid/gas interface 340 is formed between an inert gas, such asnitrogen (N2) supplied by supply tanks (not shown) connected to thepressurizer 380, and the liquid primary coolant.

In one embodiment, the external surfaces of the tubes 332 may includeintegral fins to compensate for the reduced heat transfer rates in thegaseous superheated steam media. The superheater tube bundle isprotected from erosion (i.e. by tiny water droplets that may remainentrained in the up-flowing steam) by ensuring that the steam flow iscounter-flow being parallel along, rather than across, the tubes 332 inthe tube bundle.

Referring now to FIGS. 1 and 5A, the start-up sub-system 500 of thenuclear steam supply system 100 will be described in accordance with oneembodiment of the present invention. In addition to discussing thecomponents of the start-up sub-system 500 below, the operation of thestart-up sub-system 500 in conjunction with the operation of the nuclearsteam supply system 100 as a whole will be discussed below. Prior to thestart-up processes taking place as will be discussed in more detailbelow, the primary coolant loop 190 is filled with the primary coolant,but the primary coolant is at ambient temperature and is not flowingthrough the primary coolant loop 190. Utilizing the start-up sub-system500 of the present invention, the primary coolant is heated, made toflow through the primary coolant loop 190, and then able to continuepassively flowing through the primary coolant loop 190 without the useof any pumps after disconnecting the start-up sub-system 500 from theprimary coolant loop 190.

In order to start up the nuclear steam supply system 100 and beginwithdrawing the control rods to initiate a fission chain reaction by thenuclear fuel in the reactor vessel 200, the primary coolant should beheated to a no load operating temperature, which in certain embodimentscan be between 500° F. and 700° F., more specifically between 550° F.and 650° F., and more specifically approximately 600° F. Ensuring thatthe primary coolant is at the no load operating temperature beforenormal operation (i.e., before flowing the steam to the turbine andbefore withdrawing the control rods) is beneficial for several reasons.First, it ensures that the primary coolant has a completely turbulatedflow across the fuel core while the control rods are being withdrawn,which avoids localized heating and boiling. Second, it ensures that thereactivity of the water is in the optimal range during start-up andnormal operation. Because the nuclear steam supply system 100 does notutilize any pumps to flow the primary fluid through the primary coolantloop 190 during normal operation but rather relies on thermosiphon flowas discussed above, conventional means of using frictional heat from thepumps to heat up the primary coolant is unavailable. Thus, the inventivenuclear steam supply system 100 uses the start-up sub-system 500 to heatthe primary coolant up to the no load operating temperature during startup procedures.

The start-up sub-system 500 is designed to have a high margin of safety.The start-up sub-system 500 also ensures a fully turbulent flow acrossthe fuel core in the reactor vessel 200 and heats the water to no-loadoperating temperature prior to any withdrawal of the control rods. Asdiscussed in detail above, during start-up of the nuclear steam supplysystem 100, the primary coolant is located within the primary coolantloop 190 in the reactor vessel 200 and in the steam generating vessel300, but it does not flow through the primary coolant loop 190initially. While the primary fluid is positioned in the primary coolantloop 190, the start-up sub-system 500 draws or receives a portion of theprimary coolant from the primary coolant loop 190, heats up the portionof the primary coolant to form a heated portion of the primary coolant,and injects the heated portion of the primary coolant back into theprimary coolant loop 190. Thus, the start-up sub-system 500 forms afluid flow circuit that withdraws some of the primary coolant from theprimary coolant loop 190 and heats the primary coolant prior tore-injecting that portion of the primary coolant into the primarycoolant loop 190.

When the start-up sub-system 500 injects the heated portion of theprimary coolant into the primary coolant loop 190, this initiates aventuri effect that creates fluid flow of the entire body of the primarycoolant within the primary coolant loop 190. Specifically, the injectedheated portion of the primary coolant flows within the primary coolantloop and pulls the initially static primary coolant within the primarycoolant loop 190 with it as it flows, thereby creating an entireturbulent flow of the primary coolant (including the original staticprimary coolant and the heated portion of the primary coolant) throughthe primary coolant loop 190. Furthermore, because the primary coolantinjected from the start-up sub-system is heated relative to thetemperature of the primary coolant within the primary coolant loop 190,this injection begins to heat up the primary coolant inventory withinthe primary coolant loop 190. When the primary coolant within theprimary coolant loop 190 reaches the no-load operating temperature, thestart-up sub-system 500 can be fluidly disconnected from the reactorvessel 200 and the steam generating vessel 300 and flow of the primarycoolant through the primary coolant loop 190 will continue due tothermosiphon properties.

In the exemplified embodiment, the start-up sub-system 500 comprises anintake conduit 501, a pump 502, an injection conduit 503, a heatingelement 504 and a Venturi flow effect injection nozzle 505 (alsoalternatively referred to herein as Venturi nozzle 505). The intakeconduit 501, the pump 502, the injection conduit 503 and the injectionnozzle 505 are all fluidly coupled together so that a portion of theprimary coolant that is received by the start-up sub-system 500 willflow through each of the intake conduit 501, the pump 502, the injectionconduit 503 and the injection nozzle 505. The intake conduit 501 isfluidly coupled to the suction of the pump 502 and the discharge orinjection conduit 503 is fluidly coupled to the discharge of the pump502.

In the exemplified embodiment, the entire nuclear steam supply system100 including the reactor vessel 200, the steam generating vessel 300and the start-up sub-system 500 are housed within a containment vessel400. This ensures that in the event of a loss-of-coolant accident duringstart-up, all of the high energy fluids are contained within thecontainment boundary of the containment vessel 400. The details of thecontainment vessel 400 can be found in PCT/US13/42070, filed on May 21,2013, the entirety of which is incorporated herein by reference.Furthermore, the start-up sub-system 500 is at least partiallypositioned external to the reactor vessel 200 and to the steamgenerating vessel 300. Specifically, in the exemplified embodiment whilethe intake conduit 501 is at least partially positioned within one ofthe reactor vessel 200 or the steam generating vessel 300 to draw aportion of the primary coolant into the start-up sub-system 500 and theinjection nozzle 505 is at least partially positioned within one of thereactor vessel 200 or the steam generating vessel 300 to inject theheated portion of the primary coolant back into one of the reactorvessel 200 or the steam generating vessel 300, the pump 502 and theheating element 504 are positioned entirety external to the reactorvessel 200 and to the steam generating vessel 300.

The portion of the primary coolant that is introduced into the start-upsub-system 500 flows in a single direction through the start-upsub-system 500 from the intake conduit 501 to the injection nozzle 505.The intake conduit 501 and the injection conduit 503 can be a singlepipe or conduit or can be multiple pipes or conduits that are fluidlycoupled together. In some embodiments, the intake conduit 501 and theinjection conduit 503 comprise heavy wall pipes that are sized to bebetween five and seven inches in diameter, and more specificallyapproximately six inches in diameter. Furthermore, the injection nozzle505 has a smaller diameter than the diameter of the intake conduit 501and the injection conduit 503, and can be between two and four inches,or approximately three inches. However, the invention is not to be solimited and the sizing of the intake conduit 501, the injection conduit503 and the injection nozzle 505 can be greater than or less than thenoted ranges in other embodiments.

In the exemplified embodiment, the pump 502 may be a centrifugal pumpdesigned to pump a sufficiently large flow of the primary coolant todevelop turbulent conditions in the reactor core. Specifically, incertain embodiments the pump 502 can pump approximately 10% of thenormal flow through the primary coolant loop 190 and is able to overcomeany pressure differential through the riser pipe 337. Of course, theinvention is not to be so limited and the pump 502 can be any type ofpump and can pump any amount of the primary coolant through the start-upsub-system 500 as desired or needed for start-up procedures to besuccessful. In one embodiment, the pump preferably may have a flowcapacity of less than 100% of the normal flow through the primarycoolant loop 190 because flow in the primary coolant system may be agravity driven as opposed to a pumped coolant flow system and isintended to be used for reactor start-up or shut-down operation only,not during normal reactor operating conditions.

The heating element 504 can be any mechanism or apparatus that iscapable of transferring heat into the portion of the primary coolantthat is flowing through the start-up sub-system 500. The heating element504 can be a single heater or a bank of heaters. The heating element cantake on any form, including being a resistance wire, molybdenumdisilicide, etched foil, a heat lamp, PTC ceramic, a heat exchanger orany other element that can provide heat to a liquid that is flowingthrough a conduit. In certain embodiments, the heating element 504 canbe powered by electrically powered resistance rods. In otherembodiments, the heating element 504 can be powered by and may betubular heat exchanger(s) supplied with steam by an auxiliary steamboiler. In this design, heating element 504 may be a shell and tube heatexchanger having auxiliary steam flowing through the shell side andprimary reactor coolant flowing through the tube side of the heatexchanger. Any mechanism can be used as the heating element 504 so longas the heating element 504 can transfer heat into the primary coolant inorder to heat up the portion of the primary coolant that is flowingthrough the start-up sub-system 500.

In the exemplified embodiment, the intake conduit 501 comprises an inlet506 that is located within the primary coolant loop 190. Morespecifically, in the embodiment of FIG. 1 the inlet 506 of the intakeconduit 501 is positioned at a bottom of the reactor vessel 200. Thismay include positioning the inlet 506 of the intake conduit 501 withinthe bottom flow plenum 228 of the reactor vessel 200. However, theinvention is not to be so limited and the bottom of the reactor vessel200 may include positioning the inlet 506 of the intake conduit 501adjacent to the bottom end 225 of the shroud 220. Furthermore, in otherembodiments the inlet 506 of the intake conduit 501 can be located in acentral vertical region of the reactor vessel 200 or in a top verticalregion of the reactor vessel 200 or within the steam generating vessel300 as discussed in more detail below with reference to FIGS. 5A-5C.Positioning the inlet 506 of the intake conduit 501 at the bottom of thereactor vessel 200 ensures that the portion of the primary coolant thatis removed from the primary coolant loop and received by the start-upsub-system 500 is the coolest or coldest primary coolant available inthe primary coolant loop. Such positioning of the inlet 506 of theintake conduit 501 can reduce start-up time. However, the invention isnot to be limited by positioning the inlet 506 of the intake conduit 501at the bottom of the reactor vessel 200, and other positions arepossible as discussed above and again below with regard to FIGS. 5A-5C.

Specifically, FIGS. 5A-5C show different places that the inlet 506 ofthe intake conduit 501 can be positioned in different embodiments. Thepositioning of the inlet 506 of the intake conduit 501 illustrated inFIGS. 5A-5C are merely exemplary and are not intended to be limiting ofthe present invention. Therefore, it should be understood that the inlet506 of the intake conduit 501 can be located at any other desiredlocation along the primary coolant loop. In FIG. 5A, the inlet 506 ofthe intake conduit 501 is positioned at the bottom of the reactor vessel200. In FIG. 5B, the inlet 506 of the intake conduit 501 is positionedat the bottom of the steam generating vessel 300 or within the outerflow path 272, 372 of the fluid coupling 270 between the steamgenerating vessel 300 and the reactor vessel 200. In FIG. 5C, the inlet506 of the intake conduit 501 is positioned within the riser pipe 337 orwithin the inner flow path 271, 371 of the fluid coupling 270 betweenthe steam generating vessel 300 and the reactor vessel 200. The inlet506 of the intake conduit 501 can also be positioned within the riserpipe 337 upstream of the fluid coupling 270 or at any other desiredlocation within the primary coolant loop 190. Regardless of its exactpositioning, the location of the inlet 506 of the intake conduit 501 isthe location from which the portion of the primary coolant is withdrawnfor introduction into the start-up sub-system 500.

In certain embodiments, the pump 502 may be fluidly coupled to more thanone intake conduit or more than one inlet so that the primary coolantcan be drawn from the primary coolant loop 190 and introduced into thestart-up sub-system 500 from more than one location simultaneously, orso that an operator can determine the location from which the primarycoolant can be taken based on desired applications and start-up timerequirements. Specifically, there may be multiple intake conduits thatare connected to the injection conduit such that there are valvesassociated within each intake conduit. One of the intake conduits canhave an inlet located at a bottom of the reactor vessel 200 and anotherone of the intake conduits can have an inlet located at a bottom of thesteam generating vessel 300. Thus, an operator can open one or more ofthe valves while leaving the other valves closed to determine thelocation(s) within the primary coolant loop 190 from which the primarycoolant will be drawn for introduction into the start-up sub-system 500.The multiple intake conduits with their respective isolation or shutoffvalves may be fluidly coupled to a common intake piping manifold fluidlyconnected to the suction of the pump 502. Such arrangements are wellknown to those in the art without further elaboration.

Referring back to FIG. 1, regardless of the exact positioning of theinlet 506 of the intake conduit 501, a portion of the primary coolant isdrawn from the primary coolant loop 190 into the intake conduit 501 ofthe start-up sub-system 500 when it is desired to start the nuclearsteam supply system 100. More specifically, in the exemplifiedembodiment the primary coolant is drawn from the primary coolant loop190 by the operation of the pump 502. Specifically, in the exemplifiedembodiment when the pump 502 is turned on, the portion of the primarycoolant is drawn from the primary coolant loop 190 and into the start-upsub-system 500. When the pump is turned off, none of the primary coolantis drawn from the primary coolant loop 190 and into the start-upsub-system 500.

Although the use of the pump 502 for drawing the portion of the primarycoolant into the start-up sub-system 500 is described above, theinvention is not to be so limited. In certain other embodiments, thestart-up sub-system 500 may include a shutoff or isolation valve(s) 501Apositioned at some point along the intake conduit 501. In someembodiments, the start-up sub-system 500 may also or alternativelyinclude another shutoff or isolation valve(s) 503A positioned at somepoint along the injection conduit 503. The use of valves 501A, 503Aenables the start-up sub-system to be cut off or isolated from thereactor vessel 200 and the steam generating vessel 300 from a fluid flowstandpoint. Specifically, by closing the valves the primary coolant willbe unable to enter into the start-up sub-system 500, and the primarycoolant loop will form a closed-loop path. One embodiment of the use ofvalves in the start-up sub system 500 and the connection/placement ofthose valves will be described in more detail below with reference toFIG. 7.

Where valves are used, the valves can be alterable and moved between anopen state whereby a portion of the primary coolant flows from theprimary coolant loop and into the start-up sub-system 500, and a closedstate whereby the primary coolant is prevented from flowing into thestart-up sub-system 500. In some embodiments, both the pump 502 and oneor more valves may be used in conjunction with one another to facilitateand regulate the amount of flow of the portion of the primary coolantbypassed into the start-up sub-system 500.

Still referring to FIG. 1, when the pump 502 is operating (and anyvalves positioned between the reactor vessel 200 and the start-upsub-system 500 and between the steam generating vessel 300 and thestart-up sub-system 500 are open), the portion of the primary coolantflows from the primary coolant loop 190 and into the intake conduit 501through the inlet 506. In FIG. 1, this portion of the primary coolant istaken from the bottom of the reactor vessel 200 where the primarycoolant is at its coldest. However, as discussed above the primarycoolant can be taken from any location along the primary coolant loop190, including from within the steam generating vessel 300 and withinthe riser pipe 337. The portion of the primary coolant flows through theintake conduit 501, passes through the pump 502 and flows into theinjection conduit 503 whereby the portion of the primary coolant passesthrough the heating element 504. As the portion of the primary coolantpasses through or by the heating element 504, the portion of the primarycoolant is heated and becomes a heated portion of the primary coolant.The heated portion of the primary coolant then continues to flow alongthe injection conduit 503 and into the injection nozzle 505 where theheated portion of the primary coolant is injected back into the primarycoolant loop 190.

Referring to FIGS. 1 and 6 concurrently, the injection of the heatedportion of the primary coolant into the primary coolant loop 190 will bediscussed in more detail. In the exemplified embodiment, the injectionnozzle 505 is positioned within the riser pipe 337 of the steamgenerating vessel 300. Of course, the invention is not to be so limitedand the injection nozzle 505 can be positioned at other locations withineither the reactor vessel 200 or the steam generating vessel 300 asdesired. Specifically, the injection conduit 505 can be located withinthe riser column 224 of the reactor vessel 200, within the downcomer 222of the reactor vessel 200, within the pressurizer 380 of the steamgenerating vessel 300 or at any other desired location.

In the exemplified embodiment the injection nozzle 505 is centrallylocated within the riser pipe 337 so as to be circumferentiallyequidistant from the inner surface of the riser pipe 337. Furthermore,the injection nozzle 505 faces in an upwards direction so that theheated portion of the primary coolant injected from the injection nozzle505 is made to flow in a vertical upward direction. In the exemplifiedembodiment, the injection conduit 503 enters into the steam generatingvessel 300 at the bottom-most tubesheet 333A elevation, and theinjection nozzle 505 is positioned near or at the elevation of thebottom-most tubesheet 333A. More specifically, the injection conduit 503extends horizontally into the riser 337 just below the bottom-mosttubesheet 333A, an elbow connects the injection conduit 503 to theinjection nozzle 505, and the injection nozzle 505 extends verticallyfrom the elbow within the riser pipe 337. Specifically, the injectionnozzle 505 in one embodiment is located so as to inject the heatedportion of the primary coolant just above the bottom-most tubesheet333A. Thus, in the exemplified embodiment the injection nozzle 505 islocated at and injects the heated portion of the primary coolant to alocation above the bottom plenum 338 of the steam generating vessel 300.Of course, the invention is not to be so limited in all embodiments andas discussed above the location at which the heated portion of theprimary coolant is injected can be modified as desired.

In the exemplified embodiment, the injection nozzle 505 of the start-upsub-system 500 injects a heated portion of the primary coolant(indicated with arrows as 511) into the riser pipe 337 in a firstvertical direction. At the time of the initial injection of the heatedportion of the primary coolant 511 into the riser pipe 337, the primarycoolant (indicated with arrows as 512) is positioned in the primarycoolant loop 190 including within the riser pipe 337 but is static ornon-moving. After the start-up sub-system 500 begins injecting theheated portion of the primary coolant 511 into the riser pipe 337 in thefirst vertical direction, the entire body of the primary coolant 512within the primary coolant loop 190 begins to flow in the first verticaldirection due to the venturi effect, as discussed below. In certainembodiments, once the primary coolant 512 within the primary coolantloop 190 begins to flow, it flows at a first flow rate. Furthermore, theheated portion of the primary coolant 511 is injected at a second flowrate, the second flow rate being greater than the first flow rate.

In the exemplified embodiment, the injection of the heated portion ofthe primary coolant 511 creates a venturi effect in the closed loop path190, and more specifically in the riser pipe 337. Specifically,introducing a jet of high velocity heated primary coolant 511 into theriser pipe 337 creates a venturi effect in the riser pipe 337 thatcreates a low pressure in the vicinity of the injection nozzle 505. Thisin essence creates what is also referred to in the art as a Venturi orjet pump. This low pressure pulls the primary coolant 512 from thebottom of the riser pipe 337 upwardly in the direction of the flow ofthe heated portion of the primary coolant 511 to the top of the steamgenerating vessel 300 and facilitates the flow of the primary coolantthrough the primary coolant loop 190. Thus, the injection of the heatedportion of the primary coolant 511 from the start-up sub-system 500initiates start-up of the nuclear steam supply system 100 byfacilitating the flow of the primary coolant 512 through the primarycoolant loop 190. Specifically, due to the venturi effect the mixture ofthe heated portion of the primary coolant 511 and the primary coolant512 flows upwardly within the riser pipe 337, and due to gravity themixed primary coolant 511/512 flows downwardly through the tubes 332 inthe steam generating vessel 300 and downwardly through the downcomer 222in the reactor vessel 200 due to thermosiphon flow. When the heatedportion of the primary coolant 511 mixes with the primary coolant 512 inthe riser pipe 337, this heated mixture expands and becomes less denseand more buoyant than the cooler primary coolant below it in the primarycoolant loop. Convection moves this heated liquid upwards in the primarycoolant loop as it is simultaneously replaced by cooler liquid returningby gravity.

Once the primary coolant gets heated up to the no-load operatingtemperature, the flow of the primary coolant in the primary coolant loop190 is continuous without the use of an external pump. The start-upsub-system 500 and the pump 502 associated therewith merely operate toheat up the temperature of the primary coolant and to begin the flow ofthe primary coolant in the primary coolant loop 190 and to heat up theprimary coolant in the primary coolant loop 190. However, the start-upsub-system 500 can be disconnected from the primary coolant loop 190once no-load operating temperature of the primary coolant is reached andthermosiphon flow of the primary coolant in the primary coolant loop isachieved.

As discussed above, as the primary coolant in the primary coolant loop190 heats up, the primary coolant expands. Thus, in certain embodimentsthe system 100 may be fluidly coupled to a chemical and volume controlsystem which can remove the additional volume of the primary coolant asneeded. Furthermore, such a chemical and volume control system can alsoremove dissolved gases in the primary coolant. Thus, the chemical andvolume control system can be used to control the liquid level bydraining and adding additional primary coolant into the primary coolantloop 190 as needed. In certain embodiments, the chemical and volumecontrol system may be capable of adding and/or removing the primarycoolant at a desired rate, such as at a rate of sixty gallons per minutein some embodiments. When used, the chemical and volume control systemcan be fluidly coupled to the nuclear steam supply system 100 at anydesired location along the primary coolant loop 190.

During start-up of the nuclear steam supply system 100, the start-upsub-system 500 continues to take a portion of the primary coolant fromthe primary coolant loop 190, heat the portion of the primary coolant toform a heated portion of the primary coolant, and inject the heatedportion of the primary coolant into the primary coolant loop 190. Theflow of the heated portion of the primary coolant into the primarycoolant loop 190 serves to heat up the primary coolant (which isactually a mixture of original primary coolant and the heated portion ofthe primary coolant) during the start-up process. Once the primarycoolant in the primary coolant loop 190 reaches the no load operatingtemperature, the pump 502 is turned off or the start-up sub-system 500is otherwise isolated/disconnected/valved off from the primary coolantloop 190. In certain embodiments, only after the primary coolant reachesthe no load operating temperature do the control rods begin to bewithdrawn.

During the start-up procedures discussed above, the secondary coolant(i.e., feedwater) continues to be circulated on the shellside 305 of thesteam generating vessel 300. Thus, as the primary coolant heats up dueto the start-up procedures and begins to flow through the primarycoolant loop 190 including through the tubes 332 of the steam generatingvessel, the secondary coolant flowing through the shellside 305 of thesteam generating vessel 300 boils to produce steam. This steam is heldinside of the steam generating vessel 300 until a desired pressure isreached. Once the desired pressure is reached, a steam isolation valve(i.e., a valve between the steam generating vessel 300 and the turbine900) is opened and a portion of the steam is sent to the turbine 900 forturbine heat-up and the remainder of the steam is sent to the condenserin a bypass operation.

In certain embodiments, the steam is sent to the turbine 900 for powerproduction only when all of the control rods are fully withdrawn and thenuclear steam supply system 100 is at full power. Furthermore, as notedabove the control rods are only fully withdrawn in some embodimentsafter the primary coolant reaches the no-load operating temperature.Thus, in those embodiments, during the start-up process no steam is sentto the turbine 900 for power production (although it may be sent to theturbine 900 for turbine heat-up). Power production begins in suchembodiments only when the start-up process is complete and the primarycoolant flows through the primary coolant loop 190 passively without theoperation of a pump.

In addition to heating the primary coolant within the primary coolantloop 190, the start-up sub-system 500 can also be used for draining theprimary coolant from the primary coolant loop 190 if the need arises. Incertain embodiments, such as the embodiment depicted in FIGS. 1 and 5Awhereby the inlet 506 of the intake conduit 501 is positioned at abottom of the reactor vessel 300, this can include draining primarycoolant from the reactor vessel 200. Furthermore, the start-up supplysystem 500 can be used to remove debris that may accumulate at thebottom of the reactor vessel 200 or at the bottom of the steamgenerating vessel 300, depending on the location of the inlet 506 of theintake conduit 501.

In certain embodiments, as the primary coolant is being heated byinjecting the heated portion of the primary coolant into the primarycoolant loop 190 using the start-up sub-system 500, pressure in theprimary coolant loop 190 is increased in stages by introducing highpressure inert gas into the pressurizer 380 volume. The two-phase (inertgas—water vapor with liquid water) equilibrium maintains the liquidlevel in the pressurizer 380 volume. The staged increase in pressurefollows the typical heat-up curve as shown in FIG. 8, which is based ona brittle toughness curve specific to the primary coolant loop 190,reactor vessel 200 and steam generating vessel 300 material ofconstruction.

Referring now to FIG. 7, the interconnection between the start-upsub-system 500 and the reactor vessel 200 will be described. AlthoughFIG. 7 only depicts the connection between the start-up sub-system 500and the reactor vessel 200, it should be appreciated that an identicalconnection can be used for connecting the start-up sub-system 500 to thesteam generating vessel 300. Stated another way, FIG. 7 illustrates themanner in which the intake conduit 501 is connected to the reactorvessel 200 in a manner that prevents or eliminates or substantiallyreduces the likelihood of a loss-of-coolant accident. Of course, certainembodiments may omit the valves discussed below, and in certainembodiments the connection between the start-up sub-system 500 and thereactor vessel 200 and the steam generating vessel 300 may be achievedin other manners than that discussed directly below.

As illustrated in FIG. 7, the intake conduit 501 comprises a concentricpipe construction including an inner pipe 508 that carries the portionof the primary fluid from the primary coolant loop 190 and an outer pipe509 that concentrically surrounds the inner pipe 508. The outer pipeserves as a redundant pressure boundary to contain the portion of theprimary coolant within the piping in case the inner pipe 508 were todevelop a leak. Two independent pressure enclosures (i.e., the innerpipe 508 and the outer pipe 509) serve to render the potential of a pipebreak loss-of-coolant accident non-credible.

The inner pipe 508 is directly connected to a valve 600. Furthermore,the valve 600 is enclosed in a pressure vessel 602 which encloses theentirety of the valve 600 except for the valve stem 601. Thus, the valvestem 601 extends from the pressure vessel 602 so that manual opening andclosing of the valve 600 is still possible while the pressure vessel 602remains enclosing the valve 600. The inner pipe 509 connects to thevalve 600 within the pressure vessel 602. Thus, the pressure vessel 602prevents any loss-of-coolant accident event initiating at the weldmentbetween the valve 600 and the inner/outer pipe 508, 509 arrangement.Specifically, if there was a breakage at the weldment between the valve600 and the inner pipe 508, any coolant leakage would occur within thepressure vessel 602 and would not escape into the environment orelsewhere where it could cause harm.

Furthermore, the reactor vessel 200 comprises a forging 290 extendingfrom the sidewall thereof. The valve 600 is directly welded to theforging 290. This eliminates the possibility of pipe breakage betweenthe reactor vessel 200 and the valve 600. Furthermore, the connectionbetween the forging 290 and the valve 600 occurs within the pressurevessel 602 so that a break at the weldment between the forging 290 andthe valve 600 would result in coolant leakage occurring within thepressure vessel 602.

Shutdown System for Nuclear Steam Supply System

For shutting down a typical pumped-flow pressurized water reactor inpresently designed systems which include reactor coolant pumps forcirculating coolant through the reactor vessel, it is necessary to cooldown the primary reactor coolant from hot full power conditions toshutdown cold conditions, hereafter called Cold Shutdown Condition(CSC). The fuel core can only be accessed (by opening the reactor vesselhead) to start refueling operation after reaching the Cold ShutdownCondition (CSC).

Once the reactor core has been fully shutdown by inserting all theshutdown control rods, the reactor core will begin to reject itsresidual decay heat to the primary coolant (which in this case will bepressurized water). Initially the primary coolant temperature in the hotleg of a traditional PWR is close to the normal operating temperature.The primary coolant has sufficient enthalpy (for the first few hours) toenable the steam generator to produce steam. The low pressure steam thusproduced bypasses the turbine and is sent directly to the condenserwhere it is condensed and returned back to the steam generator using thefeedwater pumps. In this manner, the decay heat is rejected to theultimate heat sink (i.e. the environment) through the use of for examplea cooling tower which cools the condenser or an air cooled condenser.Throughout the entire operation, the primary coolant is being circulatedthrough the reactor pressure vessel using the reactor coolant pump.

The decay heat being produced by the shutdown reactor core willmonotonically reduce and will reach a point (within the first fewhours), hereafter called Intermediate Switchover Condition (ISC), whereit no longer has sufficient enthalpy to enable the steam generator intoproducing steam. At this juncture, the primary coolant is routed througha set of heat exchangers called the Residual Heat Removal heat exchanger(RHR heat exchanger) where the primary coolant is cooled by rejectingits heat to the component cooling water supplied by the componentcooling water supply system.

Once the primary coolant temperature reaches the cold shutdowncondition, the reactor flange can be opened to commence the refuelingoperation.

According to another aspect of the invention, a nuclear steam supplyshutdown system 700 is provided which functions to cool down the fuelcore and dissipate residual decay heat generated by the core under steamsupply shutdown conditions so that the reactor vessel may ultimately beaccessed for maintenance, refueling, repairs, inspection, and/or otherreasons. In various embodiments disclosed herein, this is accomplishedby cooling the primary coolant and/or by cooling the secondary coolantusing cooling apparatuses fluidly coupled to flow loops locatedexternally or outside of the steam generating vessel 300 and reactorvessel 200, as further described herein.

In one embodiment, the start-up sub-system 500 may advantageously alsobe re-used in a modified reverse operating mode as part of the steamsupply shutdown system 700 for use with the passive nuclear steam supplysystem 100 that normally operates via natural gravity-driven coolantcirculation through the reactor. In lieu of heating the primary coolantduring startup of the reactor, the start-up sub-system 500 is insteadoperated to remove heat from and cool the primary coolant flowingthrough the nuclear steam supply system 100 (i.e. reactor vessel 200 andsteam generating vessel 300). As further described below, the shutdownsystem 700 is operable to facilitate shutdown of the reactor from a hotfull power normal operating state to a cold shutdown state in a safe andcontrolled manner which protects the steam supply system components fromdamage due to the associated thermal transients experienced duringreactor shutdown. The shutdown system 700 may be used of either plannedor emergency reactor shutdown situations.

By definition, a passively safe nuclear steam supply system as disclosedherein does not include or require any 100% primary coolant flow pumpsin the primary reactor coolant loop because the flow is driven bygravity, not mechanical pumps. In a passively cooled reactor, naturalcirculation flow will be sustained even after the reactor shutdowncontrol rods are fully inserted into the core. The residual decay of thespent fuel core provides the motive force to sustain the naturalcirculation flow due to buoyancy effects, albeit at a reducedcirculation rate.

The residual decay heat is a fraction of the full power heat decayingmonotonically with time, thereby reducing the natural circulation flowrate, and taking the flow eventually into the laminar regime. This ishighly undesirable as it is difficult to predict the occurrence ofnucleate boiling phenomenon at the fuel cladding surface. Departure fromnucleate boiling is a highly undesirable phenomenon in a PWR, which isbest to be avoided to ensure operational stability and performancepredictability.

Also, the cooling rate will be affected as the heat transfer coefficientis at least an order of magnitude lower in the laminar regime comparedto the turbulent regime. This increases the duration to reach ColdShutdown Conditions (CSC) delaying the refueling process. It isdesirable to reach the cold shutdown condition in as short a time aspossible.

The shutdown system 700 described below is uniquely designed to have ahigh margin of safety from the above described undesirable event and toensure quick and safe shutdown of the reactor and steam supply system100. The shutdown system ensures fully turbulent flow across the fuelcore during the cool down process to optimize core cooling.

In one exemplary embodiment shown in FIGS. 9 and 10, the steam supplyshutdown system 700 may generally include a primary coolant coolingsystem 580 configured for cooling primary coolant and a secondarycoolant cooling system 800 configured for cooling the secondary coolantwhich normally undergoes a phase change in the steam generating vessel300 during normal reactor operation (i.e. not shutdown or start-up) fromliquid to steam to power the turbine-generator set for producingelectricity. Each of the primary coolant cooling system 580 andsecondary coolant cooling system 800 are comprised of separate externalpiping loops or circuits with pump-driven flow and coolant coolingapparatuses; the former system extracting and circulating primarycoolant from the primary side (e.g. tubeside) of the steam generator 300and the latter system extracting and circulating secondary coolant fromthe secondary or steam side (e.g. shell side) of the steam generator.Interconnecting piping used in each of these foregoing sub-systems maybe made from nuclear industry standard piping of suitable diameter andwall thickness.

Primary Coolant Side Heat Removal

FIG. 9 is a schematic flow diagram showing an initial first operatingphase of the steam supply shutdown system 700 for removing and rejectingresidual decay heat from the nuclear fuel core. This situation isencountered when first shutting down the reactor, wherein the primarycoolant has enough residual heat to heat the secondary coolant to atemperature sufficient to produce steam. The secondary coolant thereforeis in a steam phase or state.

FIG. 10 is a schematic flow diagram showing a later second operatingphase of the steam supply shutdown system 700 for removing and rejectingresidual decay heat from the nuclear fuel core. This situation isencountered later in the reactor shutdown cycle, wherein the primarycoolant still heats the secondary coolant but does not have enoughresidual heat to produce steam any longer. The secondary coolant istherefore heated by the hotter primary coolant, but remains in a liquidphase or state.

Referring generally but not exclusively to FIGS. 9 and 10, the primarycoolant cooling system 580 in one embodiment may utilize and generallybe comprised of the same start-up sub-system 500 (see FIG. 1) which hasbeen slightly reconfigured for performing cooling rather than heatingthe cooling primary coolant during reactor shutdown. The start-upsub-system 500 may therefore be dual purposed which advantageouslyreduces capital equipment and maintenance costs. The primary coolantcooling system 580 therefore generally includes the same Venturi or jetpump such as Venturi injection nozzle 505 and primary coolantcirculation pump 502 of the start-up sub-system 500, which operates inthe same manner already described herein. To that basic system, however,the primary coolant cooling system 580 adds a cooling apparatus which inone embodiment may be a “dual purpose” primary coolant tubular heatexchanger 515 that replaces the heating element 504 of the start-upsub-system 500. A dual purpose heat exchanger 515 operates in both auser-selectable cooling mode (during shutdown) or a heating mode (duringstartup), as further described herein.

In other possible embodiments, it will be appreciated that completelyseparate primary coolant cooling system 580 and start-up sub-system 500may be used. Accordingly, the invention is not limited to eitherequipment arrangement.

In one arrangement, the Venturi injection nozzle 505 as alreadydescribed herein may remain located and positioned inside the straightvertical internal riser pipe 337 of steam generator 300 (see FIGS. 1, 3,6, and 9-10). The Venturi nozzle 505 may be located near and just abovethe bottom end of the straight portion of riser pipe 337 so that thenozzle discharges into the riser pipe through a majority of its length.

The Venturi injection nozzle 505 is oriented to face and dischargeprimary coolant flow vertically and upwards through the riser pipe 337parallel to vertical axis VA of steam generating vessel 300. Theinjection conduit 503 may laterally enter through the cylindrical shell312 of steam generating vessel 300 and riser pipe 337 as best shown inFIG. 18. Preferably, in one embodiment, the injection conduit 503 entersthe shell 312 of steam generating vessel 300 below the bottom tubesheet333A so as to not interfere with the vertically straight heat exchangertubes 332 mounted through the top surface of tubesheet.

A flow elbow 507 may be provided to change the flow direction ininjection conduit 503 from horizontal to vertical. The Venturi injectionnozzle 505 may be attached to the outlet of the flow elbow 507 orpreferably on a short stub pipe 510 fluidly coupled to the outlet of theelbow. The latter stub piping allows the vertical position of theVenturi injection nozzle 505 to be adjusted as desired within the steamgenerator riser pipe 337.

As already described herein, the injection conduit 503 may be formed ofheavy wall piping (e.g. 6 inches in diameter in one embodiment) thatenters the riser pipe 337 at the bottom tubesheet 333A elevation) andmay be then be reduced to a smaller bore nozzle 505 (e.g. 3″ nozzlediameter). The heated water being pumped through the reducedbore/diameter Venturi injection nozzle 505 creates a pressurized jetstream of inlet water in the riser pipe 337 which creates the Venturiflow effect to draw primary coolant out from the reactor pressure vessel200 into the riser pipe. The combined primary coolant flow from theVenturi nozzle discharge and primary coolant drawn upwards from thereactor vessel 200 rises together through the internal riser pipe 337 ofthe steam generating vessel 300 towards the pressurizer 380. The primarycoolant then reverses direction and flows back down inside the tubes 332into the reactor vessel 200, and then upwards inside riser column 224(holding the nuclear fuel core) back to the internal riser pipe 337, asalready described herein.

In one embodiment, the heating element 504 of the start-up sub-system500 may be replaced by the dual purpose shell and tube tubular heatexchanger 515 as described above if the shutdown system 700 incorporatesa modified version of start-up sub-system 500. This same heat exchangermay therefore be used for both initially heating the primary coolantduring reactor start-up as already described herein in a first operatingmode using a suitable steam source as the heating medium, and alsoconversely for removing heat from the primary coolant during reactorshutdown in a second reverse operating mode using a suitable cold watersource as the cooling medium. In one embodiment, component cooling watermay provide the cooling medium. This type of heat exchanger may also bereferred to in the art as a “dual purpose primary heater.”

During both the start-up and shutdown operation, primary coolant willflow through the tubeside (i.e. inside the tubes) in the dual purposeheat exchanger 515. However, during shutdown cooling operation as shownin FIG. 10, colder component cooling water from a component coolingwater system 950 is pumped through the shellside (i.e. outside of thetubes) while allowing the hotter primary coolant to flow inside throughthe tubeside. The colder component cooling water cools the primarycoolant flowing in the tubes of the heat exchanger 515. As describedabove, primary coolant is pumped through the heat exchanger 515 by thecirculating water pump 502 prior to introducing the coolant back intothe steam supply system 100 and reactor vessel 200 for cooling thereactor. In one embodiment, the heat exchanger 515 is disposed on thesuction side of circulating water pump 502 at a suitable location in theintake conduit 501. This arrangement allows the heat exchanger tubes tohave thinner wall thicknesses since the pressure of the primary coolantis lower on the suction side of the pump 502. In other possibleembodiments, however, the heat exchanger 515 may be disposed on thedischarge side of the circulating water pump 502 wherein thicker walledtubes would be provided for primary coolant pressure retention.

Component cooling water systems 950 are well known in the art and arepumped systems forming a continuous closed flow loop operable tocirculate cooling water to a variety of plant equipment and componentshaving cooling needs. The component cooling water extracts heat from theplant components. The heated cooling water flow is collected frommultiple plant components and cooled back down in heat exchangersprovided as part of the component cooling water system 950 which operatetypically by either water and/or air cooling. The now cooled coolingwater is then recirculated back to the plant components to repeat thecycle.

The inlet 506 of the intake piping 501 may take suction and extractprimary coolant from the reactor vessel 200 or steam generating vessel300 at any suitable location, some possible non-limiting examples ofwhich are shown in FIGS. 5A-C and described above with respect to thestart-up sub-system 500. The intake piping 501 arrangement of thestart-up sub-system 500 may therefore be identical for the primarycoolant cooling system 580. The location of the primary coolantextraction point selected from the reactor vessel 200 or steamgenerating vessel 300 will depend on a number of factors, includingwithout limitation accessibility based on the physical layout of thesteam supply system 100 equipment, thermal flow dynamics, and otherconsiderations.

In reactor and steam supply shutdown operation, a portion of the primarycoolant flowing through the reactor vessel 200 and steam generatingvessel 300 is extracted or drawn into the primary coolant cooling system580 assisted by pump 502 through the intake conduit 501. The remainingportion of the primary coolant remains in the reactor vessel 200 andsteam generating vessel 300 and continues to flow through the primarycoolant flow loop as described herein forming a circulation path insidethe steam generating vessel and reactor vessel. In one embodiment, theamount of primary coolant extracted and circulated through the start-upsub-system 500 is less than 100% of the total volume of primary coolantpresent in the reactor vessel 200 and steam generating vessel 300. Insome embodiments, the amount of extracted primary coolant may be lessthan 50%, and less than 25% of the total primary coolant volume. In oneexemplary non-limiting embodiment, the extracted primary coolant may beabout 10% of the total primary coolant volume stored in the reactorvessel 200 and steam generating vessel 300.

During the first initial operating phase of the steam system shutdownsystem 700 shown in FIG. 9 occurring right after reactor shutdown,primary coolant is extracted from the reactor vessel 200 or steamgenerating vessel 300 by the circulating water pump 502 and dischargedthrough the external piping loop or circuit of the primary coolantcooling 580. The primary coolant from pump 502 is discharged directlyinto the riser pipe 337 without flowing through the heat exchanger 515.During this initial phase, the temperature of the primary coolant maygenerally be too high to utilize the heat exchanger. The secondarycoolant cooling system 800 performs the function of cooling the primarycoolant, as described below. At this juncture, the primary coolantcooling system 580 functions primarily to induce and drive primarycoolant circulation through the primary coolant flow loop inside thereactor vessel 200 and steam generating vessel 300 under the reducedpower level of the reactor fuel core 230.

During the second operating phase of the steam system shutdown system700 shown in FIG. 10, the extracted primary coolant flows through heatexchanger 515 and is cooled in the manner already described beforereaching the inlet or suction of circulating water pump 502. The pump502 pressurizes and discharges the primary coolant through the injectionconduit 503 to the Venturi injection nozzle 505 under high velocity. Thepressure of the returned portion of the primary coolant is higher thanthe pressure of the primary coolant circulating through the reactorvessel 200 and steam generating vessel 300 in the primary coolant flowloop. It bears noting that the pump 502 therefore discharges primarycoolant at a higher pressure than at the extraction pressure of theprimary coolant from the primary coolant flow loop which is drawn intothe intake conduit 501.

The Venturi or jet pump formed by introducing a jet of high velocityprimary coolant water through Venturi injection nozzle 505 into theinternal riser pipe 337 of the steam generating vessel 300 produces themotive force necessary during reactor shutdown to circulate primarycoolant through the reactor vessel when insufficient heat is generatedby the reactor to sustain normal gravity-driven coolant flow. TheVenturi effect creates a low pressure in the vicinity of the nozzle 505thereby pulling the water from the reactor vessel 200 into the lowerportion of the internal riser pipe 337. The jet of primary coolant waterinjected via Venturi injection nozzle 505 mixes with the hot upwellingwater from the reactor vessel 200 and is pushed upwards with the highpressure water jet to the pressurizer 380 at the very top the heatexchanger stack in the steam generating vessel 300 (reference FIGS. 1,4, 9, and 10). The water then naturally flows downwards by gravitythrough the tubes 332 to the bottom of the reactor vessel completing afull cycle of primary coolant circulation. As the primary coolant flowsdown the heat exchanger stack tubes, the primary coolant cools down byrejecting heat across the tube walls secondary coolant. Preferably, theprimary coolant flow rate is sufficient to ensure a fully turbulent flowregime across the fuel core.

As the primary coolant water cools down, it should be noted that thevolume of water inventory eventually reduces and the loss may becompensated by a fresh inventory of water introduced into the primarycoolant flow loop from any suitable source, such as by a chemical andvolume control system in one non-limiting example.

Secondary Coolant Side Heat Removal

Referring generally but not exclusively to FIGS. 9 and 10, the secondarycoolant cooling system 800 includes a secondary residual heat removalheat exchanger 810, a secondary feedwater circulation pump 802, andsteam bypass condenser 820. Heat exchanger 810 may be a tubular heatexchanger including a shell and a tube bundle comprised of a pluralityof tubes inside the shell, as are well known in the art. In oneembodiment, the cooling water source for the heat exchanger 810 may bethe plant component cooling water system 950. Secondary feedwatercirculation pump 802 may be similar in type to pump 502, and in oneembodiment may be a centrifugal type pump. Any suitable type pump may beused, however, for pump 802 so long as it is operable to circulatesecondary coolant in a liquid state. Steam bypass condenser 820 may beany type of air or water cooled condenser operable to condense secondarycoolant in a steam phase from the nuclear steam supply system 100 toliquid (variously referred to in the art as condensate or feedwater). Invarious embodiments, the main plant steam condenser may be used as thebypass condenser 820 or a separate condenser may be provided to servethe sole function as the bypass condenser.

Referring to FIG. 9, isolation or shutoff valves 801A and 803A may beprovided respectively to isolate a steam bypass piping 801C of thesecondary coolant cooling system 800 from the steam generating vessel300 and to isolate the feedwater return piping 803 from the secondaryfeedwater circulation pump 802 back to the steam generating vessel. Thesecondary coolant cooling system 800 and associated piping loop may besized to handle 100% of the secondary coolant flow.

During the normal generating plant and reactor power cycle operation toproduce electricity,) bypass isolation valve 801A is closed and a mainsteam isolation/shutoff valve 801B is opened to allow superheated steam(secondary coolant) from the steam generating vessel 300 to flow to thesteam turbine 900 through the main steam piping 801 as shown in FIG. 9.Steam is extracted from the superheater section 340 of the steamgenerator at a first extraction point P1 located at the top of the steamgenerating vessel 300 near and below the pressurizer 380. In thisembodiment shown, the main condenser may be dual purposed and alsoserves as the steam bypass condenser 820. Steam (secondary coolant)flows through the steam turbine 900, is condensed by the dual purposemain/bypass condenser 820, and then is pumped back as a liquid throughfeedwater return piping 803 to the steam generating vessel 300 such asby secondary feedwater circulation pump 802 in one embodiment. Theliquid secondary coolant may be returned to the steam generator section330 (or preheater section 320 if provided) of the steam generatingvessel 300 at a return point R1.

During reactor shutdown, the secondary coolant cooling system 800 of theshutdown system 700 may be operated in two phases to cool the hotsecondary coolant; a first steam cooling phase and a subsequent secondliquid cooling phase. These phases are each described in turn below.

Referring to FIG. 9, a first initial shutdown system 700 operating phaseor mode (secondary coolant steam cooling phase) is shown in which thesecondary coolant cooling system 800 utilizes the feedwater circulationpump 802 and steam bypass condenser 820. The secondary residual heatremoval heat exchanger 810 is not used in this initial steam coolingmode.

During the first few hours following a reactor shutdown, the primarycoolant cooling system 580 using the jet pump provided by Venturi nozzle505 is operated as described above to continue to circulate primarycoolant through the reactor vessel 200 and steam generating vessel 300as described above and shown in FIG. 9. The dual purpose heat exchanger515 is bypassed and not operated in one embodiment during this initialshutdown system 700 operating mode. There is still sufficient decay heatbeing rejected to the primary coolant by the nuclear fuel core 230 inthis first shutdown system operating mode to coerce the steam generatorto convert the secondary coolant into steam. Accordingly, this residualheat picked up by the secondary coolant in the steam generating vessel300 must continue to be cooled absent normal operation of the steamturbine 900 which is not run during reactor shutdown.

To accomplish the foregoing cooling, in one embodiment the main steamisolation valve 801B is closed and bypass isolation valve 801A is openedto divert the steam flow (secondary coolant) through the steam bypasspiping 801C of the secondary coolant cooling system 800 directly to thebypass condenser 820, thereby bypassing the steam turbine 900 (referenceFIG. 9). Steam continues to be extracted from the superheater section340 at a first extraction point P1 located at the top of the steamgenerating vessel 300 near and below the pressurizer 380. This is thesame steam extraction point P1 used during normal plant power cycle andturbine operation discussed above. The steam is condensed and cooled inthe bypass condenser 820, and the collected water condensate flowsthrough suction piping 804 to the inlet of the secondary feedwatercirculation pump 802. Pump 802 pressurizes the condensate (liquidsecondary coolant) which is pumped back to the steam generator vessel300 through feedwater return piping 803 forming a continuous closedcirculation flow loop (external to the reactor and steam generatingvessels 200, 300) which cools and gradually reduces the temperature ofthe primary coolant. This also replenishes the lost inventory ofsecondary coolant water in the steam generating vessel 300 and cools thesecondary coolant to remove the residual heat transferred by the reactorfuel core 230 to the primary coolant, which in turn is transferred tothe secondary coolant in the steam generator. Accordingly, the secondarycoolant ultimately extracts and rejects residual heat from the reactorcore in conjunction with operating the primary coolant shutdown system580 in the manner described above. The cooled liquid phase secondarycoolant is returned at return point R1 to the steam generating section330 or preheater section 320 if provided.

After the first few hours when the Intermediate Switchover Condition(ISC) is reached, the decay heat from the reactor fuel core 230 is nolonger sufficient to convert the secondary coolant into steam in thesteam generating vessel 300. Referring to FIG. 10, the secondary coolantwater remains in liquid phase and reaches a normal water level in thesteam generating vessel 330 near the top of the intermediate steamgenerator section 330, which may be diametrically enlarged in someembodiments. Further cooling of the liquid secondary coolant is stillrequired using the secondary residual heat removal heat exchanger 810 inlieu of the bypass condenser 820 to reach temperature conditions in thereactor suitable for a full shutdown.

Referring now to FIG. 10, a second shutdown system 700 operating phaseor mode (secondary coolant liquid cooling phase) is initiated to furthercool the secondary coolant to a level commensurate with final reactorshutdown conditions (i.e. primary coolant reaches the cold shutdowncondition). A new secondary coolant extraction point P2 near the top ofthe steam generating section 330 of the steam generating vessel 300 isused to capture the heated secondary coolant flowing upwards through theshell side of the vessel which is still being heated by the residualheat in the primary coolant. Extracted hot secondary coolant (liquidphase) flows through secondary coolant suction conduit 807 and an openisolation valve 807A directly into the inlet of secondary feedwatercirculation pump 802. It should be noted that the pump inlet source fromthe bypass condenser 820 via suction piping 804 is not used in thispresent secondary coolant cooling phase and may be isolated by closingisolation valve 804A. The hot liquid secondary coolant (water) isdischarged from pump 802 and flows through feedwater return piping 803to secondary residual heat removal heat exchanger 810. Heat exchanger810 cools the hot secondary water by rejecting its heat to the componentcooling water supplied by the component cooling water system 950 asalready described herein. The now cold secondary coolant flows throughthe remainder of feedwater return piping 803 to return point R1 in thesteam generator vessel 300. The foregoing secondary coolant circulationflow loop is continued until the reactor has been cooled sufficientlyfor complete shutdown.

FIG. 11 is a graph showing an exemplary decay heat curve for reactorcore 230 of the nuclear steam supply system 100. In this non-limitingexample, core decay heat may reach minimum levels within approximately24 hours which may be compensated for by using the steam supply shutdownsystem 700 described herein.

The shutdown system 700 may be configured to minimize or eliminateexposure to a Loss-of-Coolant Accident (LOCA) in this system. Referringto FIG. 7, the interconnecting piping may be made of a double wallconstruction. This pipe arrangement consists of a two-concentric-pipeconstruction with the inner pipe carrying the fluid while the outer oneserves as a redundant pressure boundary to contain the fluid within thepiping in case the inner pipe were to develop a leak. Two independentpressure enclosures, thus designed, serve to render the potential of apipe-break LOCA non-credible. All isolation valves may be directlywelded to the vessel nozzle forgings (see, e.g. FIG. 7) minimizing thepossibility of pipe breakage between the pressure vessel and the valve.The isolation valves themselves may be enclosed in a small removablepressure vessel (called a stuffing box) as shown which encloses theentirety of the valve except for the sealed valve stem. This containsand prevents any LOCA event initiating at the weldment between valve andsteam generator vessel and/or double walled piping.

The shutdown system 700 may further be configured to provide filtration.In the intake conduit 501 arrangement of the primary coolant coolingsystem 580 (start-up sub-system 500) shown in FIG. 5A, the intake pipingreaches all the way to the bottom of the reactor vessel 200 bottom headand may be used as a siphon for debris removal. The debris, if any,generally consists of corrosion products and it is known from reactoroperating experience that all debris tends to accumulate at the bottomhead of the reactor pressure vessel. When the primary coolant reaches afew degrees below maximum operating temperature of the filtration systemduring steam system shutdown, the filtration is turned on and thecirculating pump 502 will draw and extract the debris through the intakepiping 501 which may include a filtration system disposed upstream ofthe circulating pump 502. The temperature is set by the maximumoperating temperature of the filtration system. The filtration systemmay comprise a set of mechanical filters and a demineralizer. However,the debris could be radiologically active due to long periods ofresidence near the fuel core. Therefore the filtration system may belocated in a heavily shielded part of the reactor containment.

Unless otherwise specified, the components described herein maygenerally be formed of a suitable material appropriate for the intendedapplication and service conditions. All conduits and piping aregenerally formed from nuclear industry standard piping. Componentsexposed to a corrosive or wetted environment may be made of a corrosionresistant metal (e.g. stainless steel, galvanized steel, aluminum, etc.)or coated for corrosion protection.

While the invention has been described with respect to specific examplesincluding presently preferred modes of carrying out the invention, thoseskilled in the art will appreciate that there are numerous variationsand permutations of the above described systems and techniques. It is tobe understood that other embodiments may be utilized and structural andfunctional modifications may be made without departing from the scope ofthe present invention. Thus, the spirit and scope of the inventionshould be construed broadly as set forth in the appended claims.

What is claimed is:
 1. A nuclear steam supply system with startupprimary coolant heating system, the nuclear steam supply systemcomprising: a reactor vessel having an internal cavity; a reactor corecomprising nuclear fuel disposed within the internal cavity and operableto heat a primary coolant; a steam generating vessel fluidly coupled tothe reactor vessel; a riser pipe positioned within the steam generatingvessel and fluidly coupled to the reactor vessel; a primary coolant loopformed within the reactor vessel and the steam generating vessel, theprimary coolant loop being configured for circulating primary coolantthrough the reactor vessel and steam generating vessel; and a primarycoolant heating system comprising: an intake conduit having an inletfluidly coupled to the primary coolant loop; a pump fluidly coupled tothe intake conduit, the pump configured and operable to extract andpressurize primary coolant from the primary coolant loop and dischargethe pressurized primary coolant through an injection conduit; theinjection conduit having an outlet positioned inside the riser pipe toinject the pressurized primary coolant into the riser pipe from thepump; and a heat exchanger configured and operable to heat thepressurized primary coolant.; an injection nozzle fluidly coupled to theoutlet of injection conduit for injecting the heated pressurized primarycoolant into the riser pipe.
 2. The nuclear steam supply systemaccording to claim 2, wherein the injection nozzle is a Venturi nozzlewhich discharges the heated pressurized primary coolant verticallyupwards into the riser pipe to induce flow of the primary coolantthrough the primary coolant loop.
 3. The nuclear steam supply systemaccording to claim 1, wherein the inlet of the intake conduit is locatedat a bottom of the reactor vessel.
 4. The nuclear steam supply systemaccording claim 1, wherein the inlet of the intake conduit is located ata bottom of the steam generating vessel.
 5. The nuclear steam supplysystem according to claim 1, wherein the inlet of the intake conduit islocated at a bottom of the riser pipe.
 6. The nuclear steam supplysystem according to claim 1, wherein the steam generating vesselcomprises a steam generating section and a superheater section disposedabove the steam generating section in vertically stacked relationship,the superheater section being operable to heat a secondary coolant tosuperheated steam conditions.
 7. The nuclear steam supply systemaccording to claim 6, further comprising: the steam generating sectionand the superheater section including a pair of vertically spaced aparttubesheets and a tube bundle comprising a plurality ofvertically-oriented tubes extending between the tubesheets; and whereinthe injection nozzle is positioned so as to inject the pressurizedprimary coolant into the riser pipe of the steam generating section ator near an elevation of a bottom one of the tubesheets.
 8. The nuclearsteam supply system according to claim 1, wherein the injection conduitis a pipe having a diameter of approximately six inches and wherein theinjection nozzle has a diameter of approximately three inches.
 9. Thenuclear steam supply system according to claim 1, wherein injecting thepressurized primary coolant into the riser pipe through the injectionnozzle creates a low pressure Venturi effect that causes the primarycoolant to flow through the primary coolant loop.
 10. The nuclear steamsupply system according to claim 1, further comprising: the steamgenerating vessel including a plurality of stacked heat exchangersfluidly connected in a vertically stacked relationship; the stacked heatexchangers each including a pair of vertically spaced apart tubesheetsand a tube bundle comprising a plurality of vertically oriented tubesextending between the tubesheets; wherein upon injecting the pressurizedprimary coolant into the riser pipe, the primary coolant flowsvertically upwards through the riser pipe to a top of the steamgenerating vessel, vertically downwards from the top of the steamgenerating vessel through the tubes of the stacked heat exchangers,vertically downwards through a downcomer in the reactor vessel to thebottom of the reactor vessel, vertically upwards within a riser columnin the reactor vessel, and from the riser column in the reactor vesselback into the riser pipe in the steam generating vessel; and wherein asecondary coolant flows upwards between the tubes on a shell side ofeach of the stacked heat exchangers.
 11. The nuclear steam supply systemaccording to claim 1, wherein the reactor vessel, the steam generatingvessel, and the primary coolant heating system are positioned within acontainment vessel.
 12. The nuclear steam supply system according toclaim 1, wherein at least a portion of the primary coolant heatingsystem is positioned external to the reactor vessel and the steamgenerating vessel.
 13. The nuclear steam supply system according toclaim 1, wherein the primary coolant heating system is a one-way fluidflow circuit in which the primary coolant flows from the primary coolantloop through the intake conduit, through the pump, into the injectionconduit, through the heat exchanger, further through the injectionconduit, and into the riser pipe of the steam generating vessel via theinjection nozzle.
 14. The nuclear steam supply system according to claim1, wherein the primary coolant heating system further comprises a valvethat is fluidly coupled to the reactor vessel by a piping nozzle at oneend and at another end to an inner pipe fluidly coupled to the valve andarranged inside an outer pipe that concentrically surrounds the innerpipe, the inner and outer pipes collectively forming the intake conduit.15. The nuclear steam supply system according to claim 1, wherein theprimary coolant heating system extracts a portion of the total volume ofthe primary coolant from the primary coolant loop, the remainder of theprimary coolant remaining in the primary coolant loop.
 16. The nuclearsteam supply system according to claim 13, further comprising a pressurevessel enclosing the valve, a valve stem of the valve protruding fromthe pressure vessel, and wherein a connection between the inner pipe andthe valve and a connection between the valve and the integral pipingnozzle of the reactor vessel are located within the pressure vessel. 17.The nuclear steam supply system according to claim 1, wherein thepressurized primary coolant from the injection nozzle mixes with theprimary coolant drawn into the riser pipe from the reactor vessel by aVenturi flow effect to form a mixed primary coolant flow through theriser pipe.
 18. The nuclear steam supply system according claim 1,wherein the primary coolant is cooled in the heat exchanger by waterfrom a component heating system.
 19. The nuclear steam supply systemaccording to claim 1, wherein the heat exchanger is a shell and tubetype.
 20. A nuclear steam supply system with startup heating system, thenuclear steam supply system comprising: a reactor vessel having aninternal cavity; a reactor core comprising nuclear fuel disposed withinthe internal cavity and operable to heat a primary coolant; a steamgenerating vessel fluidly coupled to the reactor vessel and containing asecondary coolant for producing steam to operate a steam turbine, thesteam generating vessel including a superheater section and a steamgenerating section; a riser pipe positioned inside the steam generatingvessel and fluidly coupled to the reactor vessel; a primary coolant flowloop formed within the reactor vessel and the steam generating vessel,the primary coolant flow loop being configured and operable forcirculating primary coolant through the reactor vessel and steamgenerating vessel; a primary coolant startup heating system comprising:a first pump having an inlet fluidly coupled to the primary coolant flowloop, the first pump configured and operable to extract and pressurize aportion of the primary coolant from the primary coolant loop; a Venturiinjection nozzle having an inlet fluidly coupled to a discharge of thefirst pump and an outlet positioned inside the riser pipe in the steamgenerating vessel, the injection nozzle receiving and injecting thepressurized portion of the primary coolant into the riser pipe from thepump; and a heating element operable to heat the extracted primarycoolant prior to injecting the pressurized portion of the primarycoolant into the riser pipe.
 21. The nuclear steam supply systemaccording to claim 20, wherein the heating element is a shell and tubeheat exchanger configured to receive auxiliary steam from a steam sourceother than the steam generator which flows through a shell side, theprimary coolant flows through a tube side of the heat exchanger and isheated by the steam.
 22. The nuclear steam supply system according toclaim 20, wherein the steam generating vessel and reactor vessel arevertically elongated, the superheater section and steam generatingsection of steam generating vessel being disposed in vertically stackedrelationship and the superheater section being positioned above thesteam generating section.
 23. The nuclear steam supply system accordingto claim 20, wherein the portion of the primary coolant extracted by thepump is less than 50% of the total volume of primary coolant containedin the primary coolant flow loop.
 24. A nuclear steam supply system withstartup system, the nuclear steam supply system comprising: a reactorvessel having an internal cavity; a vertically elongated reactor corecomprising nuclear fuel disposed within the internal cavity and operableto heat a primary coolant; a vertically elongated steam generatingvessel fluidly coupled to the reactor vessel and containing a secondarycoolant for producing steam to operate a steam turbine, the steamgenerating vessel including a superheater section and a steam generatingsection; a vertically elongated riser pipe positioned inside the steamgenerating vessel and fluidly coupled to the reactor vessel; a primarycoolant flow loop formed within the reactor vessel and the steamgenerating vessel, the primary coolant flow loop being configured andoperable for circulating primary coolant through the reactor vessel andsteam generating vessel; a secondary coolant flow loop formed outside ofthe reactor vessel and steam generating vessel, the secondary coolantflow loop being configured and operable for circulating secondarycoolant through the steam generating vessel; and a Venturi jet pumpdisposed inside the riser pipe of the steam generating vessel, the jetpump including an injection nozzle fluidly coupled to the primarycoolant flow loop by a pump fluidly coupled to the primary coolant flowloop which extracts and pressurizes a portion of the primary coolantfrom the primary coolant flow loop and discharges the pressurizedportion of the primary coolant to the injection nozzle; wherein the jetpump receives and injects a portion of the primary coolant into theriser pipe which draws and mixes primary coolant from the reactor vesselwith the injected portion of the primary coolant in the jet pump tocirculate the primary coolant through the primary coolant flow loop. 25.The nuclear steam supply system according to claim 24, furthercomprising a first heat exchanger disposed upstream of the jet pump, theheat exchanger configured and operable to heat the portion of theprimary coolant received by the jet pump before injection into the riserpipe.
 26. The nuclear steam supply system according to claim 24, whereinthe injection nozzle discharges primary coolant in an upwards directionsinside the riser pipe.